The traditional approach to protecting relatively complex integrated circuits (ICs) is to package them in multilayer ceramic packages. There are a number of problems associated with this approach. These packages are expensive, they take a long time to tool, and they generally have poor electrical performance.
Many of these problems are addressed by recently developed Plastic Pin Grid Array (PPGA) packages. PPGAs enjoy a number of benefits over ceramic packages: they are relatively inexpensive, they may be quickly and inexpensively tooled, and they have excellent electrical performance.
The primary problem with PPGAs, which has slowed their wide acceptance, is their lack of hermeticity. That is, unlike metal, glass, or ceramic materials, they do not form a complete barrier to moisture and gases. Consequently, they tend to be less reliable than hermetic packages. This problem is especially pronounced in non-computer room environments where the air may have a high humidity or high ionic content.
To achieve the required protection of an IC in a PPGA, a number of solutions have been proposed. These solutions include placing a coated gel over the IC. It is widely recognized that a thick and adherent gel provides protection for the surface of an IC. However, there are problems associated with this approach. Specifically, most gels have much higher expansion coefficients than the IC. As a result, during temperature cycling, the encapsulants tend to rip bond wires off the bond pads of the IC.